Freeze & Thaw Cycle - Tackling road defects in the north west of Scotland12 Feb 2018

BEAR Scotland is doing all it can to carry out repairs across trunk roads in the North East and North West of Scotland, after harsh weather conditions in December, January and February has had a detrimental effect on sections of the network.

John Wrigley, BEAR Scotland’s North West Roads Manager, explains the science behind the cause of this deterioration across the road surface, and what BEAR teams are doing to carry out repairs as soon as possible.

A road pavement has a natural life cycle and, at some point, it will require maintenance, repair and replacement. While most of these repair and replacement projects can be planned, sometimes a road surface can fail prematurely. Such failure can happen over the course of months, or it can be very sudden.

Asphalt surfaces, such as the ones that make up most of the trunk roads BEAR manage, are known for their durability and resilience. Its strengths make it a highly used material for many road surfacing applications and is by far the preferred material for the vast majority of roads across the UK, and indeed Europe. Like all paved surfaces however, it is susceptible to deterioration due to the laws of Mother Nature. Despite the great longevity of a properly laid asphalt surface, it can be cut short simply due to long term exposure to the elements.

Freeze and Thaw Cycle

Deterioration of constructed road pavement is natural. It is natural because over time the materials that make up the road pavement begin to break down and become affected by elements such as temperature (especially freezing followed by thawing), rain, sunlight and chemicals (such as diesel) that come into contact with the road’s surface.

The asphalt binder that is the “glue” of the road begins to lose its natural resistance to water, allowing it to penetrate into and underneath the pavement. Once this happens, the surface can quickly fall prey to a number of different types of deterioration.

Due to the expansion and contraction of water when it freezes and thaws, the ice formed in freezing conditions expands in the voids within the carriageway’s matrix which can lead to tearing of the binder which holds the aggregate in the road together. This generally happens at surface level where the frost penetrates the upper layers. With the repeated expansion and contraction the stones in the road surface begin to loosen as their bond is weakened. The more this happens, the greater the rate of deterioration. This is a major contributory factor into how potholes occur.

In a section of road where surfacing materials are all of the same age and composition, sudden onset deterioration can occur on a rapidly spreading basis and water penetrates not only from the running surface down, but also through the looser stone which occurs on the sides of the pothole as it forms.

This freeze / thaw action has happened across the North West network at an extraordinary rate over this winter period due to the range of temperatures fluctuation above and below the freezing point. In January alone this freeze and thaw cycle occurred very frequently with 22 days across the month where this was recorded - a higher number of instances than in recent years.

The timing and length of the recent freeze – which saw 28 continuous days between December and January where temperatures dropped below zero – has made matters worse than in previous years.

On the A85 in Connel, one of the areas that was badly damaged by the weather earlier this year, road surface temperatures in December ranged from a high of 10.3 degrees to a low of -6.5 degrees - a swing of nearly 17 degrees – which, when coupled with heavy rain in the same month, has meant that the road surface had begun to break down in some places due to this flux in temperature and weather conditions.

Usually the weather for creating potholes arrives in February and March, when there is more rain, however rainfall through January has been higher than in previous years. Unfortunately, this too has exacerbated the impact to the network.

Challenges

It’s physically difficult to repair potholes at the very time when they become most prevalent. When temperatures are low, it is almost impossible to work with hot asphalt because it cools in transit from the plants where it’s made to the often remote parts of the network where it is needed. Even trucks that are specially equipped with heated compartments can only reach so many potholes in the course of a workday to undertake the repairs to the road system.

Additionally, fixing potholes in winter is difficult due to moisture: snow, ice and water naturally collect in the holes and cracks. The existing pavement needs to be dry for most asphalt mixtures to “tack” to form a solid, permanent bond. If moisture remains in the hole it can start the deterioration process all over again: freezing, expanding and allowing room for more precipitation to enter and expand further. When air temperatures have dropped to below minus 12 degrees during January this year, it is a real challenge to try to repair the road surfaces in winter conditions.

As a result, and due to the accelerated rate of pothole development in the last few months, the fallback solution used to overcome this has been to place temporary repair material into holes to minimize the spread of potholes in the short term while a permanent solution can be delivered when warmer and drier conditions prevail.

That’s why spring and summer are the ideal months for conducting road repairs where possible, while at the same time we have to consider the impact of our work and limit disruption at all times so as to minimise any impact on the important tourist economy.

What BEAR are doing to carry out road repairs

BEAR Scotland are urging people to report potholes. This can usually be done through the website here , or by calling Traffic Scotland’s Freephone trunk road customer care line on 0800 028 1414.